Attack may be trivial, but compromising private keys of majority of stakeholders looks a tad bit harder.
Would you consider the risk of compromising only 7-12 stakeholders who likely know each other and work together (thus compromising one would likely lead to compromising multiple) a secure arrangement for a currency ?
P.S....What is funny about all this is Nxt was already attacked in a fundamental way even before being released and thus has little hope of widespread adoption. Speaking about the security and viability of PoS variants is one thing , but IMHO Nxt was doomed from the start. Bitshares seems to have taken a dangerous recent precedent as well with the "merger" which is effectively switching the currency from a deflationary one to an inflationary one.